Providing image data

ABSTRACT

Embodiments of the present invention provide a method of providing image data for constructing an image of a region of a target object ( 40 ) comprising providing incident radiation from a radiation source at a target object ( 40 );detecting, by at least one detector ( 50 ), a portion of radiation scattered by the target object; and providing image data via an iterative process responsive to the detected radiation ( 340 ), wherein said iterative process comprises estimating a wave front of scattered radiation at a plane of the detector, updating a portion of the wave front based on the detected radiation, leaving a portion of the estimated wave front substantially unchanged, setting a portion of the wave front, corresponding to a portion of radiation scattered by the target object and not detected by the detector, to one or more values, and providing image data based upon the updated wave front.

BACKGROUND

The present invention relates to method and apparatus for providingimage data from which an image of at least a portion of a target objectmay be generated.

WO 2005/106531, which is incorporated herein by reference for allpurposes, discloses a method and apparatus of providing image data forconstructing an image of a region of a target object. Incident radiationis provided from a radiation source at the target object. An intensityof radiation scattered by the target object is detected using at leastone detector. The image data is provided responsive to the detectedradiation. A method for providing such image data via an iterativeprocess using a movable softly varying probe function such as atransmittance function or illumination function is also disclosed. Themethods and techniques disclosed in WO 2005/106531 are referred to as aptychographical iterative engine (PIE).

PIE provides for the recovery of image data relating to at least an areaof a target object from a set of diffraction pattern measurements.Several diffraction patterns are recorded at a measurement plane usingone or more detectors, such as a CCD or the like. A probe function,which might be a transmittance function associated with a post-targetobject aperture or an illumination function, must be known or estimated.

WO 2010/064051, which is incorporated herein by reference for allpurposes, discloses an enhanced PIE (ePIE) method wherein it is notnecessary to know or estimate the probe function. Instead a process isdisclosed in which the probe function is iteratively calculated step bystep with a running estimate of the probe function being utilised todetermine running estimates of an object function associated with atarget object.

Other methods of providing image data based on measurement of scatteredradiation are also known.

It is an object of embodiments of the invention to at least mitigate oneor more of the problems of the prior art.

SUMMARY OF THE INVENTION

According to aspects of the present invention there is provided methodsand apparatus as set forth in the appended claims.

According to an aspect of the invention there is provided a method ofproviding image data for constructing an image of a region of a targetobject, comprising providing incident radiation from a radiation sourceat a target object; detecting, by at least one detector, a portion ofradiation scattered by the target object; and providing image data viaan iterative process responsive to the detected radiation, wherein saiditerative process comprises estimating a wave front of scatteredradiation at a plane of the detector, updating a portion of the wavefront based on the detected radiation, leaving a portion of theestimated wave front substantially unchanged, setting a portion of thewave front, corresponding to a portion of radiation scattered by thetarget object and not detected by the detector, to one or more values,and providing image data based upon the updated wave front.

The detecting step may optionally comprise detecting a portion ofradiation scattered by the target object with the incident radiation oran aperture at first and second positions. Further positions of theaperture or incident radiation may be utilised.

The portion of the estimated wave front substantially unchanged mayoptionally be relatively larger than the portion of the wave front setto one or more values. Relatively larger may be at least the same sizeor in some embodiments a predetermined number of times larger. Thepredetermined number of times may be at least twice as large or at leastfour times as large. That is, the area of the estimated wave front setto one or more values may be less than 50%, 25%, 10%, 5% or 1% of thearea which is updated.

The portion updated based on the detected radiation may be around 5-15%of the total wave front. The portion of the estimated wave frontsubstantially unchanged may be around 85-95% of the total wave front.The portion of the wave front set to one or more values may be around5-0.5% of the total wave front. As an example, the area updated may be6% of the total wave front, the area left unchanged may be 93% and thearea set to one or more values may be around 1%, although it will beappreciated that other values may be chosen.

The one or more values may comprise predetermined values or valuesdetermined according to one or more functions.

The portion of the wave front corresponding to the portion of radiationscattered by the target object and not detected by the detector may be aperipheral region of at least a part of the wave front. The peripheralregion extends a plurality of pixels into the wave front. The pluralityof pixels may be 1, 2, 8 or 16 pixels. The plurality may be less than 16pixels.

According to another aspect of the present invention there is provided amethod of providing image data for constructing an image of a region ofa target object, comprising providing incident radiation from aradiation source at a target object, detecting, by at least onedetector, a portion of radiation scattered by the target object with theincident radiation or a post-target aperture at first and secondpositions, and providing image data via an iterative process responsiveto the detected radiation, wherein in said iterative process image datais provided corresponding to a portion of radiation scattered by thetarget object and not detected by the detector.

Optionally, the method may comprise detecting an intensity of radiationscattered by the target object with the incident radiation or the posttarget aperture at the first position with respect to the target object;re-positioning the incident radiation or the post-target aperture at atleast one further position relative to the target object; andsubsequently detecting the intensity of radiation scattered by thetarget object with the incident radiation or post-target aperture at theat least one further position.

Optionally, the method may comprise: estimating an object functionindicating at least one characteristic of said region of the targetobject; and/or estimating a probe function indicating at least onecharacteristic of incident radiation at the target object or thepost-target aperture; and iteratively re-estimating each of the objectfunction and/or probe function.

Optionally, the method may comprise multiplying the estimated objectfunction by the estimated probe function to thereby provide an exit wavefunction; propagating the exit wave function to provide an estimate ofan expected scattering pattern; and correcting at least onecharacteristic of said expected scattering pattern according to adetected intensity of radiation scattered by the target object.

Optionally, the method may comprise inverse propagating a correctedexpected scattering pattern to thereby provide an updated exit wavefunction.

Optionally, the method may comprise propagating the estimated probefunction to provide an estimate scattering pattern in a measurementplane of the detector where the propagation operator

suitably models the propagation between the plane of the object and themeasurement plane.

may comprise a Fourier Transform or a Fresnel Transform.

Optionally, the method may comprise updating a running estimate of theprobe function and/or a running estimate of an object functionsimultaneously with each iteration.

Optionally, the method may further comprise providing an initialestimate of the probe function as a prior modeled probe function. Theinitial estimate of the probe function may be provided by a randomapproximation for the probe function.

Optionally, wherein the target object may be at least partiallytransparent to the incident radiation and detecting an intensity ofradiation scattered by the target object may comprise detecting anintensity of radiation transmitted by the target object.

Optionally, the target object may be at least partially reflective tothe incident radiation and detecting an intensity of radiation scatteredby the target object may comprise detecting an intensity of radiationreflected by the target object.

According to a still further aspect of the present invention there isprovided an apparatus for providing image data for constructing an imageof a region of a target object, comprising at least one detector fordetecting an intensity of radiation scattered by the target object; anda processing means arranged to provide image data via an iterativeprocess responsive to the detected radiation object with incidentradiation or an aperture at first and second positions, wherein in saiditerative process image data is provided corresponding to a portion ofradiation scattered by the target object and not detected by thedetector.

According to a yet further aspect of the present invention there isprovided a computer program which, when executed by a computer, performsa method of providing image data for constructing an image of a regionof a target object, comprising providing incident radiation from aradiation source at a target object, detecting, by at least onedetector, a portion of radiation scattered by the target object with theincident radiation or an aperture at first and second positions, andproviding image data via an iterative process responsive to the detectedradiation, wherein in said iterative process image data is providedcorresponding to a portion of radiation scattered by the target objectand not detected by the detector. The computer program may be stored ona computer readable medium.

According to a yet further aspect of the present invention there isprovided a computer program which, when executed by a computer, performsa method of providing image data for constructing an image of a regionof a target object according to an aspect of the invention.

Embodiments of the invention provide image data having an improvedresolution. Some embodiments of the invention improve a rate ofconvergence of a method of determining the image data. Some embodimentsof the invention reduce a noise present in the image data.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of exampleonly, with reference to the accompanying figures, in which:

FIG. 1 shows an apparatus according to an embodiment of the invention;

FIG. 2 shows a detector in relation to scattered radiation according toan embodiment of the invention;

FIG. 3 shows a method according to an embodiment of the invention;

FIG. 4 illustrates a diffraction pattern according to an embodiment ofthe invention;

FIG. 5 shows images generated from image data produced by embodiments ofthe invention;

FIG. 6 shows further images generated from image data produced byembodiments of the invention; and

FIG. 7 illustrates an apparatus according to an embodiment of theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates an apparatus 100 according to an embodiment of theinvention. The apparatus is suitable to provide image data of an objectwhich may, although not exclusively, be used to produce an image of atleast a region of the object.

The apparatus 100 comprises a diffuser 10, an aperture 20, a pair oflenses 30 a, 30 b and a radiation detector 50 for detecting incidentradiation.

A radiation source, which although not shown in FIG. 1, is a source ofradiation 60 which falls upon the diffuser 10 and eventually illuminatesa region of a target object 40. It is to be understood that the termradiation is to be broadly construed. The term radiation includesvarious wave fronts. Radiation includes energy from a radiation source.This will include electromagnetic radiation including X-rays, emittedparticles such as electrons. Other types of radiation include acousticradiation, such as sound waves. Such radiation may be represented by awave function Ψ(r). This wave function includes a real part and animaginary part as will be understood by those skilled in the art. Thismay be represented by the wave function's modulus and phase. Ψ(r)* isthe complex conjugate of Ψ(r) and Ψ(r) Ψ(r)*=|Ψ(r)|² where |Ψ(r)|² is anintensity which may be measured for the wave function.

The diffuser 10 is provided to introduce a range of phase gradients intothe radiation 60 i.e. to randomise a phase of the radiation. Thediffuser 10 may be formed from many materials. In some embodiments, thediffuser 10 may be formed by one or more layers of plastic film. Asingle layer of plastic film may be used as a weak diffuser, whilst twoor more layers may be used to form a stronger diffuser. The diffuser 10contributes to a spreading of a diffraction pattern beyond an extent ofthe detector 50, as will be explained. A distance z between the targetobject 40 and the detector 50 may thus depend on the strength of thediffuser 10. For example, for the weak diffuser a distance of z=86 mmmay be used, whereas for the stronger diffuser a distance z=94.4 mm maybe used, although it will be realised that these are merely exemplarydistances. In the embodiments described with reference to FIG. 1, thediffuser 10 is located prior to, i.e. to be illuminated before, theprobe 20. However, the diffuser 10 may be arranged in other alternativelocations, such as following i.e. downstream of the probe 20, such asimmediately adjacent the probe 20 or further downstream.

The inclusion of the diffuser 10 into the apparatus 100 also leads to areduction in a dynamic range of a resultant diffraction pattern. As mostdetectors have a limited dynamic range, reducing the dynamic range ofthe diffraction pattern may allow a more faithful representation of thediffraction pattern to be determined. Furthermore, as the radiationincident on the sample 40 is spread more evenly over all diffractionangles, the incident flux required to provide the image data may bereduced, thereby reducing the possibility of causing damage to thetarget object 40.

Any type of diffuser 10 having an arbitrary transfer function may beused. As will be understood by the skilled person, the choice ofdiffuser will depend on the properties of the radiation used, and thedesired diffusion effect. For example, for visible light the diffusermay comprise a ground glass diffuser.

It will also be realised that embodiments of the invention may also beenvisaged which do not include the diffuser 10.

The aperture 20 together with the pair of lenses 30 a, 30 b in theembodiment shown in FIG. 1 form a probe which is arranged to illuminatea region of the target object 40. The lenses 30 a, 30 b shown in FIG. 1are arranged in a 4f configuration. It will be realised that embodimentsof the invention may be envisaged which do not include the pair oflenses 30 a, 30 b, include only one lens or include other arrangementsof one or more lenses.

The illuminating radiation incident on the target object 40 represents aprobe function P(r) which forms an illumination function. P(r) is thecomplex stationary value of this wave field calculated at the plane ofthe object 40.

It will be understood that rather than weakly (or indeed strongly)focusing illumination on the target 40, unfocused radiation can be usedwith a post target aperture. In this embodiment aperture is located posttarget object to thereby select a region of the target forinvestigation. The aperture is formed in a mask so that the aperturedefines a “support”. A support is an area of a function where thatfunction is not zero. In other words outside the support the function iszero. Outside the support the mask blocks the transmittance ofradiation. The term aperture describes a localised transmission functionof radiation. This may be represented by a complex variable in twodimensions having a modulus value between 0 and 1. An example is a maskhaving a physical aperture region of varying transmittance.

Incident radiation thus falls upon the up-stream side of the targetobject 40 and is scattered by the target object 40 as it is transmitted.The target object 40 should be at least partially transparent toincident radiation. The target object 40 may or may not have somerepetitive structure. Alternatively the target object 40 may be whollyor partially reflective in which case a scattering pattern is measuredbased on reflected radiation.

A wave is thus formed as an exit wave function of radiation afterinteraction with the object 40. O(r) represents a two-dimensionalcomplex object/specimen function so that each point in O(r), where r isa two-dimensional coordinate, has associated with it a complex number.We assume that O(r) will physically represent an exit wave that wouldemanate from the object which is illuminated by a plane wave. Forexample, in the case of electron scattering, O(r) would represent thephase and amplitude alteration introduced into an incident wave as aresult of passing through the object of interest. The aperture 20provides a probe function P(r) (or transmission function) which selectsa part of the object exit wave function for analysis. It will beunderstood that rather than selecting an aperture a transmission gratingor other such filtering function may be located downstream of the objectfunction. The probe function P(r−R) is an illumination wave or anaperture transmission function where the illumination wave or apertureis at a position R. The probe function can be represented as a complexfunction with its complex value given by a modulus and phase whichrepresent the modulus and phase alterations introduced by the probe intoa perfect plane wave incident up it. It will also be understood thatboth the probe and specimen functions may be three-dimensional complexfunctions, P(s) and O(s), so that each point in P(s) and O(s), where sis a three-dimensional coordinate, has associated with it a complexnumber.

The exit wave function ψ(r, R) is an exit wave function of radiation asit exits the object 40. This exit wave ψ(r, R) forms a diffractionpattern ψ(u) at a diffraction plane. Here r is a vector coordinate inreal space and u is a vector coordinate in diffraction space.

It will be understood that if the diffraction plane at which scatteredradiation is detected is moved nearer to the specimen 40 then Fresneldiffraction patterns will be detected rather than Fourier diffractionpatterns. In such a case the propagation function from the exit waveψ(r, R) to the diffraction pattern Ψ(u) will be a Fresnel transformrather than a Fourier transform. It will also be understood that thepropagation function from the exit wave ψ(r, R) to the diffractionpattern Ψ(u) may be modeled using other transforms.

In order to select the region of the target object 40 to be illuminatedor probed, the aperture 20 and lenses 30 a, 30 b may be mounted upon anx/y translation stage which enables movement of the probe function withrespect to the object 40. However, it will also be realised that theobject 40 may be moved with respect to the probe function. The probefunction may be moved by the translation stage in a grid arrangement ofpositions. The grid may comprise 20×20 positions, although other numbersof positions may be used and, furthermore, the grid may not compriseequal numbers of positions in both x and y directions. A random offsetmay be introduced into a location of each grid position. For example, ifthe grid positions have a pitch of 30 μm, the offset may be ±5 μm.Advantageously, this avoids problems associated with “raster gridpathology”, as is appreciated by those skilled in the art.

The detector 50 is a suitable recording device such as a CCD camera orthe like. The detector 50 allows the detection of the diffractionpattern in the diffraction plane. The detector may comprise an array ofdetector elements, such as in a CCD.

In embodiments of the invention the apparatus 100 is arranged to causethe scattered radiation 45 to extend beyond the aperture of the detector50 in the diffraction plane, as shown in FIGS. 1 and 2. Although FIG. 1shows the extent of the scattered radiation 45 extending verticallybeyond the detector 50, the radiation may also or alternatively extendhorizontally beyond the detector 50, as shown in FIG. 2. As will beappreciated, some of the scattered radiation is not detected by thedetector 50.

FIG. 2 schematically illustrates an extent of scattered radiation 45 inthe diffraction plane and the aperture of the detector 50. The extent ofthe scattered radiation 45 beyond the aperture of the detector 50 isillustrated in FIG. 2 with dashed arrows. Whilst the scattered radiation45 and detector 50 are shown as having a square cross-section, it willbe realised that this is merely exemplary for the purposes ofillustration and that other profiles, such as rounded, rectangular ornon-regular shapes are envisaged. Furthermore, it is also contemplatedthat the radiation may not extend beyond a periphery of the detector 50.For example, the detector 50 may comprise a central region which doesnot record an intensity of the radiation. Thus, the detector 50 onlyrecords a diffraction pattern corresponding to a portion of thescattered radiation. An intensity of a portion of the scatteredradiation is unrecorded by the detector 50. Embodiments of the presentinvention allow image data to be generated corresponding to theunrecorded radiation.

FIG. 3 illustrates a method according to an embodiment of the invention.The method illustrated in FIG. 3 involves simultaneous, step-by-stepupdating of both probe and object function estimates. However, it willalso be realised that embodiments of the invention may be envisaged inwhich only the object function may be updated and a known probe functionmay be used, as in the methods and apparatus disclosed by WO2005/106531, for example. Furthermore, in other embodiments of theinvention, a known object function may be used and the method maydetermine the probe function. It will also be appreciated that theobject function and/or probe function may be updated by other methods.

The method utilises a set s(j) of J diffracted intensities ordiffraction patterns, I_(j)(u), recorded by the detector 50, whereI_(j)(u)=|Ψ_(j)(u)|² is the square of the modulus of the wave frontincident upon the detector 50. Within each iteration of the method anestimate of the probe and object functions is updated for each of the Jdiffraction patterns measured by the detector 50. An order ofconsidering each of the J measured intensities is chosen. The order maybe numerically sequential i.e. j=1, 2, 3 . . . J. In this case,beginning with diffraction pattern s(j) and progressing through to s(J)updated estimates of the probe P₁(r) . . . P_(j)(r) and object O₁(r) . .. O_(J)(r) are produced. However, considering the diffraction patternsin a raster fashion (each pattern in a row sequentially and each rowsequentially) may cause problems particularly in relation to theestimate of the probe function drifting during the method. Therefore, insome embodiments, the diffraction patterns may be considered in a randomor other pseudo-random order. However, for the purposes of explanation,a sequential ordering of the set s(j) will be considered.

Prior to a first (k=1) iteration of the method, initial probe P₀(r) 311and object O₀(r) 312 functions are selected. The initial probe andobject functions 311, 312 may be predetermined initial values, such asinitial guesses i.e. pre-calculated approximations, randomdistributions, or may be based on other initial measurements or priorcalculations. The functions are modeled at a number of sample points andare thus represented by matrices. Such matrices can be stored andmanipulated by a computer or other such processing unit. Aptly thesample points are equally spaced and form a rectangular array.

In step 320 an exit wave ψ_(j)(r) is determined by multiplying thecurrent object and probe functions. For the first (k=1) iteration of themethod, for the first probe position s(1), the initial probe P₀(r) andobject functions O₀(r) are multiplied to determine the first exit waveΨ₁(r). For subsequent iterations of the method, the currently selectedi.e. O_(j)(r)P_(j)(r) are multiplied to determine the current exit waveψ_(j)(r).

In step 330 the exit wave ψ_(j)(r) is propagated to a measurement planeof the detector 50. The propagation produces an estimate Ψ_(j)(u) of thewavefront at the plane of the detector 50. The exit wave is propagatedto the measurement plane by a suitable transform T, as shown inequation 1. In some embodiments, the transform T may be a Fouriertransform, although in other embodiments the transform may be a Fresnelfree space propagator. It is also envisaged that other transforms may beused which are suited to the particular application of the method.

Ψ_(j)(j)=T[ψ(r)]  Equation 1

In step 340 a portion of the wavefront Ψ_(j)(u) at the plane of thedetector 50 is updated based on the measured diffraction patternU_(s(j))(u). As explained in the cited references, since Ψ_(j)(u) is acomplex-value, it may be written as shown in equation 2:

Ψ_(j)(u)=A _(j)(u)exp(iφ _(j)(u))   Equation 2

In some embodiments, the wavefront Ψ_(j)(u) at the plane of the detector50 may be considered to extend over cM×cN pixels where c is a constantvalue, for example c may equal 4 and the central M×N pixels correspondto the area of the detector 50. A modulus of this central region of thewavefront may be replaced with equation 3:

√{square root over (I_(s(j))(u))}{square root over(I_(s(j))(u))}  Equation 3

whilst the values of at least some of the remaining pixels or matrixpositions are left unchanged. In other words, the values of at leastsome of the remaining matrix positions or pixels are allowed to “float”.

In some embodiments, a step 350 includes multiplying values ofpredetermined pixels in Ψ_(j)(u) by a tapering function. In someembodiments, the tapering function results in setting a value of aperipheral region of the diffraction pattern Ψ_(j)(u). The value may bea predetermined value such as 0, although one or more other values maybe envisaged, as will be explained. The tapering function prevents noisebeing introduced into the image data caused, in some embodiments, byFourier repeats encroaching into the diffraction pattern.

FIG. 4 a illustrates the diffraction pattern Ψ_(j)(u) denoted generallyas 400. A central region 410 of the diffraction pattern is updated, asexplained above, with reference to the measured diffraction pattern. Aregion 420 surrounding the updated region 410 is allowed to float i.e.these values are not updated. A further peripheral region 430surrounding the floating region is clamped by being set to have one ormore predetermined values, or values determined according to apredetermined function. The values may, in some embodiments, be between0 and 1 i.e. an outer periphery of the region 430 may be set to 0,whilst inwardly extending pixels values may be set to values between 0and 1 i.e. inwardly approaching 1. The values may approach 1 accordingto a predetermined function. The pixel values may approach 1 in aunidirectional manner i.e. continuously increasing, either linearly ornon-linearly. Whilst values of 0 and 1 have been used, it is realisedthat these are merely exemplary. FIG. 4 b illustrates a section throughthe diffraction pattern Ψ_(j)(u) along line A-A in which the outerperipheral region is set to have a value of 0. Whilst the floatingregion 420 is illustrated as having a single constant value i.e. allpixels are set to the same value, such as 1, this is merelyillustrative.

The width of the clamped region 430 may be chosen to reduce noise at theexpense of the resolution of the image data. In some embodiments, theregion 430 may inwardly extend by one pixel i.e. the border region 430may be one pixel wide. However, in other embodiments, the region 430 maybe more than one pixel wide, such as 8 or 16 pixels wide.

In step 360 the diffraction pattern Ψ_(j)(u) is reverse propagated backto a plane of the object 40. The inverse propagation is performedaccording to the reverse of the transform used in step 330. In someembodiments, the transform used in step 360 is an inverse Fouriertransform, although as previously explained other transforms may beused. The inverse transform is performed according to Equation 4:

ψ′_(j)(r)=T ⁻¹[Ψ_(j)(u)]  Equation 4

In steps 370 and 375 the probe and object function are updated. Theupdating provides an improved probe P_(j+1) and object guess O_(j+1)(r).The updating may be performed as described in the incorporated referenceWO 201/064051, or by any other method. As described in WO 201/064051,the object function may be updated according to Equation 5 and the probefunction according to Equation 6:

$\begin{matrix}{{O_{j + 1}(r)} = {{{Oj}(r)} + {\alpha \; \frac{P_{j}^{*}(r)}{{{P_{j}\; (r)}}_{{ma}\; x}^{2}}( {{\psi_{j}^{\prime}(r)} - {\psi_{j}(r)}} )}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

The parameter α governs the rate of change of the object guess. Thisvalue may be adjusted between 0 and 2 as higher values may lead toinstability in the updated object guess. According to embodiments of thepresent invention the probe function is reconstructed in much the samemanner as the object function. Aptly the probe function guess is carriedout concurrently with the update of the object guess. (It will beappreciated that the probe Function could optionally be updated moreoften or less often than the object Function).

$\begin{matrix}{{P_{j + 1}(r)} = {{P_{j}(r)} + {\beta \; \frac{O_{j}^{*}(r)}{{{O_{j}(r)}}_{{ma}\; x}^{2}}( {{\psi_{j}^{\prime}(r)} - {\psi_{j}(r)}} )}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

The result of this update function generates the running estimate forthe probe function. The parameter β governs the rate of change of theprobe guess. This value may be adjusted between 0 and 2 as higher valuesmay lead to instability in the updated probe guess.

In step 380 it is determined whether every probe position for thecurrent iteration has been addressed. In other words, it is determinedin some embodiments whether j=J. If the current probe position is notthe last probe position of the current iteration, then the next probeposition is selected. The next probe position may be selected in step385 by j=j+1. However, if the current probe position is the last probeposition for the current iteration, then the method moves to step 390.

In step 390 it is determined whether a check condition is met. In someembodiments, the check condition may be determining whether the currentiteration number k is a predetermined value, such as k=100 i.e.determining whether a predetermined number of iterations have beenperformed. Whilst this check is computationally easy, it takes noaccount of the accuracy of the image data. Therefore, in someembodiments, the check condition compares a current estimate of thediffraction pattern against that recorded by the detector 50. Thecomparison may be made considering a sum squared error (SSE) as inequation 7:

$\begin{matrix}{{SSE} = \frac{( {{{\Psi_{j}(u)}}^{2} - {I_{s{(j)}}(u)}} )^{2}}{N}} & {{Equation}\mspace{14mu} 7}\end{matrix}$

Where N is a number of pixels in the array representing the wavefunction. The method may end when the SSE meets one or morepredetermined criteria, such as being below a predetermined value.

If the predetermined criteria is not met, then the method moves to step395 in preparation for a next iteration (k=k+1) where the probe positionis reset i.e. the first probe position is reselected, such as j=1.

As explained above, during each iteration a plurality of probe positionsare considered. Each probe position may not always correspond exactly toa pixel in the matrix that represents the image of the object O(r). Forexample, the probe does not always move a distance corresponding to anexact pixel position of the matrix that represents the image O(r) i.e.it may move to a position in between pixel locations. The matrix thatrepresents the image O(r) has a pixel pitch of 1.5 μm, whereas the probeposition may be moved at a resolution as low as, for example 0.1 μm andbe arranged to move between 30 μm grid positions with a random offsetbetween ±5 μm. Therefore it will be appreciated that the probe may moveto a location in-between pixel locations of the matrix that representsthe image O(r). In order to improve convergence of the estimateddiffraction pattern with that measured by the detector 50, someembodiments of the invention include a compensation for sub-pixelshifting of the probe location, as will be explained.

In order to include sub-pixel shift compensation for the probe functionP_(j)(r), prior to the exit wave function estimate ψ_(j)(r) beingdetermined in step 320 by multiplication of the current probe and objectfunctions, the sub-pixel shifting of the probe function is considered.In some embodiments, a linear phase ramp φ_(i)(u) is added to atransform of the current probe function P_(j)(r). The transform may be aFourier transform of the probe function.

The linear phase ramp φ_(j)(u) is, in some embodiments, determined byconsidering the distance of the probe function from a nearest pixelposition of the matrix representing the object O(r) by Equation 8:

$\begin{matrix}{{\varphi_{j}(u)} = {{- 2}{\pi ( {\frac{s_{({x,j})}u}{cM} + \frac{s_{({y,j})}v}{cN}} )}}} & {{Equation}\mspace{14mu} 8}\end{matrix}$

Where c is a constant value, for example c may equal 4 and M and N arethe number of rows and columns of pixels in the rectangular detector 50.u=[u, v] is a 2-dimensional vector with components u and v. s_((x, j))is the distance of the probe function in the direction x from thenearest pixel position in the matrix representing the object O(r) ands_((y, j)) is the distance of the probe function in the direction y fromthe nearest pixel position in the matrix representing the object O(r).

Following the addition of the phase ramp, the resultant probe functionis inversely transformed and multiplied with the current object functionto produce the current exit wave ψ_(j)(r).

In embodiments compensating for sub-pixel shifting of the detector, theupdating of the object function is performed by Equation 9:

$\begin{matrix}{{O_{j + 1}(r)} = {{{Oj}(r)} + {\alpha \; \frac{P_{j}^{*}( {r - \lbrack {s_{({x,j})},s_{({y,j})}} \rbrack} )}{{{P_{j}( {r - \lbrack {s_{({x,j})},s_{({y,j})}} \rbrack} )}}_{{ma}\; x}^{2}}( {{\psi_{j}^{\prime}(r)} - {\psi_{j}(r)}} )}}} & {{Equation}\mspace{14mu} 9}\end{matrix}$

and the updating of the probe function in step 370 is performed byEquation 10

$\begin{matrix}{{P_{j + 1}( {r - \lbrack {s_{({x,y})},s_{({y,j})}} \rbrack} )} = {{P_{j}( {r - \lbrack {s_{({x,j})},s_{({y,j})}} \rbrack} )} + {\beta \; \frac{O_{j}^{*}(r)}{{{O_{j}(r)}}_{{ma}\; x}^{2}}( {{\psi_{j}^{\prime}(r)} - {\psi_{j}(r)}} )}}} & {{Equation}\mspace{14mu} 10}\end{matrix}$

Following updating of the probe function, a conjugated linear phase ramp−φ_(j)(u) is applied to the transform of the updated probe function, inthe same way as it was applied. In some embodiments, applying theconjugated phase ramp and the application of the next sub-pixel shift tothe following probe position may be combined to avoid excessivetransforms of the probe function.

FIG. 5 shows the effect of considering the sub-pixel shift of the probe.FIG. 5( a) shows an image produced from image data resulting from 150iterations of the ePIE method disclosed in WO 2010/064051 withoutincluding sub-pixel shifting of the probe. FIG. 5( b) shows an imageproduced from image data of the same target object from 150 iterationsof the ePIE method according to an embodiment of the invention includingthe effect of sub-pixel shifting of the probe. It can be observed thatby including the effect of sub-pixel shifting of the probe noise in theimage data is decreased and a resolution of the image data is increased.The scale bar shown in the image is 100 μm long.

FIG. 6 illustrates an improvement in resolution of image data producedby embodiments of the present invention. FIG. 6( a) is an image producedfrom image data based upon a central region of the diffraction patternused to produce the images shown in FIG. 5. FIG. 6( a) is produced bythe ePIE method disclosed in WO 2010/064051. The finest resolvedfeatures are in group 5, element 2 having a resolution of 36line-pairs/mm (lp/mm), which is consistent with a spatial frequencyextent of the detector used (33 lp/mm). FIG. 6( b) illustrates an errormetric which compares an extrapolated region of a diffraction pattern,i.e. that outside of the detector, compared to recorded diffractionpattern intensity according to equation 11:

$\begin{matrix}{E = \frac{\sum\limits_{j = 1}^{J}{\sum\limits_{u}{{S(u)}{{\sqrt{I_{s{(j)}}(u)} - {{\Psi_{j}(u)}}}}^{2}}}}{\sum\limits_{j = 1}^{J}{\sum\limits_{u}{{S(u)}{I_{s{(j)}}(u)}}}}} & {{Equation}\mspace{14mu} 11}\end{matrix}$

where S(u) is 1 in the extrapolated region of the diffraction patternsand 0 on their constrained border and in the area where measured datawas used.

FIG. 6( c) is an image based on image data produced by an embodiment ofthe present invention without high spatial frequency restriction, i.e.no clamping of a border region, and without sub-pixel shifting of theprobe. The corresponding error metric is indicated in FIG. 6( b) withcircles.

FIG. 6( d) is an image based on image data produced by an embodiment ofthe present invention without high spatial frequency restriction, i.e.no clamping of a border region, but with sub-pixel shifting of theprobe. The corresponding error metric is indicated in FIG. 6( b) withsquares.

FIG. 6( e) is an image based on image data produced by an embodiment ofthe present invention with high spatial frequency restriction, i.e.including clamping of a border region, but without sub-pixel shifting ofthe probe. The corresponding error metric is indicated in FIG. 6( b)with triangles.

FIG. 6( f) is an image based on image data produced by an embodiment ofthe present invention with both high spatial frequency restriction andsub-pixel shifting of the probe. The corresponding error metric isindicated in FIG. 6( b) with crosses.

All embodiments of the invention performed 1000 iterations of therespective method to produce the image data on which FIGS. 6( c)-(f) arebased. As can be noted from FIG. 6( b) by using both the high spatialfrequency constraint and sub-pixel shifting the error does not diverge.In all cases, the resolution of the image data is improved over thatshown in FIG. 6( a). The increase in resolution for the exemplary imagedata is around 2.24 from 36 to 80.6 lm/mm. As can be appreciated fromFIG. 6( c) which includes neither high spatial frequency constraint norsub-pixel shifting, considerable noise is present which is due toFourier repeats. A similar degree of noise is present in FIG. 5( d)which includes sub-pixel shifting, although the clarity of this image isimproved due to the sub-pixel shifting. FIGS. 5( e) and (f) show howconstraining the border of the diffraction pattern significantly reducesthe noise, with FIG. 6( f) showing reduced noise and improved clarity.

FIG. 7 illustrates an apparatus 700 according to an embodiment of theinvention. The apparatus 700 is arranged to determine image data for anobject. The image data may, in some embodiments, be used to generate avisible image of the object. The visible image may, for example, beoutput to a display device.

The apparatus 700 comprises a detector 710 for detecting an intensity ofradiation falling thereon. The detector 710 corresponds to the detector50 shown in FIG. 1 arranged to record a diffraction pattern formed byradiation scattered by the target object. The detector may comprise aplurality of detecting elements each capable of outputting a signalindicative of the intensity of radiation falling thereon. The detectormay be a CCD device, or similar. The detector 710 is communicablycoupled to a processing unit 720 which is arranged to determine theimage data based on the radiation intensity detected by the detector710. The processing unit 720 comprises a memory 730 and a data processor740, such as a CPU. Although FIG. 7 shows the processing unit 720comprising one memory, the processing unit 720 may comprise two or morememories. Furthermore, although shown as comprising one data processor,the processing unit 720 may comprise more than one data processor 740,and each data processor may comprise one or more processing cores. Thememory 730 may be arranged to store measured radiation intensity dataI_(s(j))(u) corresponding to a plurality of probe positions. The dataprocessor 740 may implement a method according to an embodiment of theinvention, such as that shown in FIG. 3 and previously described. Thedata processor may store determined image data in the memory 730.

It will be appreciated that embodiments of the present invention can berealised in the form of hardware, software or a combination of hardwareand software. Any such software may be stored in the form of volatile ornon-volatile storage such as, for example, a storage device like a ROM,whether erasable or rewritable or not, or in the form of memory such as,for example, RAM, memory chips, device or integrated circuits or on anoptically or magnetically readable medium such as, for example, a CD,DVD, magnetic disk or magnetic tape. It will be appreciated that thestorage devices and storage media are embodiments of machine-readablestorage that are suitable for storing a program or programs that, whenexecuted, implement embodiments of the present invention. Accordingly,embodiments provide a program comprising code for implementing a systemor method as claimed in any preceding claim and a machine readablestorage storing such a program. Still further, embodiments of thepresent invention may be conveyed electronically via any medium such asa communication signal carried over a wired or wireless connection andembodiments suitably encompass the same.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings), may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of any foregoingembodiments. The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed. The claims should not be construed to cover merely theforegoing embodiments, but also any embodiments which fall within thescope of the claims.

1. A method of providing image data for constructing an image of aregion of a target object, comprising: providing incident radiation froma radiation source at a target object; detecting, by at least onedetector, a portion of radiation scattered by the target object; andproviding image data via an iterative process responsive to the detectedradiation, wherein said iterative process comprises estimating a wavefront of scattered radiation at a plane of the detector, updating aportion of the wave front based on the detected radiation, leaving aportion of the estimated wave front substantially unchanged, setting aportion of the wave front, corresponding to a portion of radiationscattered by the target object and not detected by the detector, to oneor more values, and providing image data based upon the updated wavefront.
 2. The method of claim 1, wherein the detecting step comprisesdetecting a portion of radiation scattered by the target object with theincident radiation or an aperture at first and second positions.
 3. Themethod of claim 1, wherein the portion of the estimated wave frontsubstantially unchanged is relatively larger than the portion of thewave front set to one or more values.
 4. The method of claim 1, whereinthe one or more values comprise predetermined values.
 5. The method ofclaim 1, wherein the one or more values comprise values determinedaccording to one or more functions.
 6. The method of claim 1, whereinthe portion of the wave front corresponding to the portion of radiationscattered by the target object and not detected by the detector is aperipheral region of at least a part of the wave front.
 7. The method ofclaim 6, wherein the peripheral region extends a plurality of pixelsinto the wave front.
 8. The method of a claim 1, further comprising:estimating at least one of an object function indicating at least onecharacteristic of at least a region of the target object and a probefunction indicating at least one characteristic of incident radiation atthe target object or the aperture; estimating the wave front at theplane of the at least one detector based on the object function and theprobe function.
 9. The method of claim 1, wherein the updating of theportion of the wave front comprises updating a modulus of the wave frontaccording to the detected radiation.
 10. The method of claim 1, whereinthe portion of the estimated wave front is updated based on the detectorradiation according to:√{square root over (I_(s(j))(u))}{square root over (I_(s(j))(u))} whereI_(s(j))(u) is a detected intensity of radiation for a probe positions(j).
 11. The method of claim 1, wherein the updated portion of the wavefront based on the detected radiation is generally centrally located.12. The method of claim 1, comprising determining an ordering of thepositions of the incident radiation or the aperture for the measurementsof the portion of scattered radiation to be used in the iterativeprocess.
 13. The method of claim 12, wherein the determined ordering israndom or pseudo-random.
 14. The method of claim 12, wherein theordering is a non-sequential pattern.
 15. The method of claim 2,comprising determining a value of a sub-pixel shift of the incidentradiation or the aperture between the first and second positions. 16.The method of claim 15, comprising applying a phase ramp to a transformof the probe function based on the sub-pixel shift value.
 17. The methodof claim 1, comprising causing radiation scattered by the target objectto extend beyond an aperture of the at least one detector.
 18. Themethod of claim 17, wherein the causing comprises arranging a diffuserto interact with the radiation, such that the scattered radiationextends beyond an aperture of the at least one detector.
 19. The methodof claim 18, wherein the diffuser is arranged in a path of the radiationpre-target or in a path of the scattered radiation.
 20. An apparatus forproviding image data for constructing an image of a region of a targetobject, comprising: at least one detector for detecting an intensity ofradiation scattered by the target object; and a processing meansarranged to provide image data via an iterative process responsive tothe detected radiation wherein said iterative process comprisesestimating a wave front of scattered radiation at a plane of thedetector, updating a portion of the wave front based on the detectedradiation, leaving a portion of the estimated wave front substantiallyunchanged, setting a portion of the wave front, corresponding to aportion of radiation scattered by the target object and not detected bythe detector, to one or more values, and providing image data based uponthe updated wave front.
 21. The apparatus of claim 20, comprising meansfor moving incident radiation or an aperture between at least first andsecond portions.
 22. The apparatus of claim 20, wherein the one or morevalues comprise values determined according to one or more functions.23. The apparatus of claim 20, wherein the one or more values comprisepredetermined values.
 24. The apparatus of claim 20, wherein the portionof the wave front corresponding to the portion of radiation scattered bythe target object and not detected by the detector is a peripheralregion of at least a part of the wave front.
 25. The apparatus of claim20, further wherein the processing means is arranged to: estimate atleast one property of an object function indicating at least onecharacteristic of at least a region of the target object and/or a probefunction indicating at least one characteristic of incident radiation atthe target abject or the aperture; estimating the wave front at theplane of the at least one detector based on the object function and theprobe function.
 26. The apparatus of claim 20, wherein processing meansis arranged to update a modulus of the portion of the wave frontaccording to the detected radiation,
 27. The apparatus of claim 26,wherein the portion of the wave front is replaced with:√{square root over (I_(s(j))(u))}{square root over (I_(s(j))(u))} whereI_(s(j))(u) is a detected intensity of radiation for a probe position j.28. The apparatus of claim 20, wherein the updated portion of the wavefront based on the detected radiation is generally centrally located.29. The apparatus of claim 20, comprising a diffuser arranged tointeract with the radiation, such that the scattered radiation extendsbeyond an aperture of the at least one detector.
 30. The apparatus ofclaim 29, wherein the diffuser is arranged in a path of the radiationpre-target or in a path of the scattered radiation.